710 STUDIES IN GENERAL PHYSIOLOGY 
rhythmically when immersed in the solution of salts with a 
monovalent kation—I obtained contractions in Na, Li, Rb, 
and Cs salts—but that the addition of a small quantity 
of a bivalent kation—Ca, Mg, Sr, Be, Mn, Co— inhibits 
these rhythmical contractions.’ This seemed to be a direct 
contradiction to the statement that calcium salts are the 
‘“‘cause”’ of the heart-beat. The significance of the calcium 
had to be looked for, then, in another direction. It was 
soon found that the muscle, the apex of the heart, and a 
Medusa contract rhythmically in a pure sodium-chloride so- 
lution, but that they soon come to a standstill. If, however, 
a trace of a soluble calcium salt is added to the sodium- 
chloride solution, the contractions continue much longer. I 
concluded from this that the pure sodium-chloride solu- 
tion acts, in the long run, as a poison—that is to say, brings 
about definite, but at present unknown, physical changes in 
the protoplasm—but that a trace of a calcium salt anni- 
hilates this toxic action. The amount of calcium neces- 
sary for this antitoxic effect is, of course, much smaller than 
the amount necessary to inhibit the rhythmical contractions. 
Soon after I succeeded in demonstrating conclusively the 
poisonous effect of a pure sodium-chloride solution, and the 
annihilation of this effect by calcium.” The eggs of a 
marine fish (Fundulus) develop normally in sea-water, but 
they can develop just as well, as I had previously found, in 
distilled water. The addition of ions from the outside is 
consequently not necessary to the development of this animal. 
I found, now, that if the freshly fertilized eggs of this fish 
are put into a pure sodium-chloride solution having a con- 
centration equal to the concentration of the sodium chloride 
in the sea-water (about $m), not a single egg can develop 
into an embryo. If, however, a trace of a calcium salt is 
1 Part I, p. 518, 
2 Part II, p. 559; Archiv fir die gesammte Physiologie, Vol. LXXX (1900), p. 229 
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